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Hypothetically, can a black hole be made of light? In general relativity, energy bends spacetime, so the type of energy shouldn't make any difference. How could this happen? Well, perhaps by shooting insanely powerful future lasers into a dot or by a merger of two neutron stars, one of matter and the other of antimatter, annihilating in the process into photons, but already after the event horizon is formed.

Regardless of how it's formed, can a black hole be made of light? My concern is that photons move with the speed of light and don't interact with each other, so they cannot be absorbed without charged particles present. Charged pairs may be created, but would eventually annihilate anyway. Can a singularity be formed from photons only?

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  • $\begingroup$ I'm not an expert on this subject, but to the best of my knowledge black holes are the result of an extremely large mass in an extremely small volume. Since photons have no mass, no amount of them could make a black hole. $\endgroup$ – Dave Coffman Sep 7 '17 at 1:17
  • $\begingroup$ @Dave Coffman: It is actually a large amount of energy in a small volume. Then E=mc2, you know. Also, photons do have mass, just not rest mass, but the type of mass shouldn't matter. $\endgroup$ – safesphere Sep 7 '17 at 1:32
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    $\begingroup$ What you're describing is a Kugleblitz. There's a YouTube video on the subject. Making Kugleblitzs is something you can (in principle) do with your Dyson Swarm, when your civilization finally gets around to making it :-). $\endgroup$ – StephenG Sep 7 '17 at 1:42
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    $\begingroup$ @StephenG: Thanks for your insight! It explains why Freeman Dyson looks like from a different world ;) img.purch.com/h/1400/… $\endgroup$ – safesphere Sep 7 '17 at 2:33
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In General Relativity it is energy that creates gravity, not just mass (rest energy). So yes forming a black hole of only photons is a theoretical possiblity.

I think you are correct that photons alone could not commence formation of a black hole in the current universe in naturally occuring conditions.

A non gravitational mechanism of achieving the energy density to create an event horizon would need to be employed. An immense array of lasers focused on single point would have to fire simultaineously.

Would a black hole thus formed be comprised of photons? No. It has to be considered that beyond the event horizon, and approaching a hypothesised singularity, the original form this energy came in becomes irrelevant, matter will adopt the approriate form according to its "temperature" due to proximity to the singularity. So we would need to place the energy (focus the lasers) to form a sphere the right radius from the singularity to have an energy density that would allow it to all remain photonic (r could be calculated I think).

(Noting that observations beyond the event horizon are in current theory impossible, what would be the point - behaviour would be indistinguishable from any other black hole to external observers).

Once an event horizon has formed, from the perspective of the external observer, a black hole grows to envelop more matter, rather than absorbing matter. (Time stops at the event horizon.) This matter/energy, then, probably (no observations made) maintains the density it had at the event horizon and may not be photonic. So, even if it was originally formed using photons, and by some mechanism the matter content remained photonic, if non photonic matter was enveloped the purity of the black hole would be permentantly disrupted.

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  • $\begingroup$ Your answer essentially repeats the background of my question. However, my particular concern is that there is nothing there to absorb the photons. Conceptually, photons can exchange virtual gravitons, but this would not stop the photons from flying. Now, if photons decay into gravitons (produce a graviton pair), then the gravitons must fly away with the speed of light, so the problem remains. $\endgroup$ – safesphere Sep 7 '17 at 4:58
  • $\begingroup$ Not really, if an event horizon forms the photons inside it will stop due to time dilation, and those outside it will either orbit in the photosphere due to the space-time curvature, or beyond that escape. They don't need to interact to be contained. $\endgroup$ – JMLCarter Sep 7 '17 at 12:49
  • $\begingroup$ I just don't get the physical meaning of photons stopping without absorption. Stopping them with a time dialation isn't too easy. In the frame of the free fall, the speed of light remains constant through the event horizon. At the stationary frame near the event horizon, the speed of light actually increases. Also the blueshift creates energy conservation concerns. Thanks for your insight! $\endgroup$ – safesphere Sep 7 '17 at 20:45
  • $\begingroup$ Stopping them with time dilation is inevitable at the event horizon for a distant observer, which is the most practical location for an observer. It means the light appear to remain in the same position to that observer. Perhaps your answer lies in improving understanding of time dilation... Also read up on photon spheres around black holes possibly might help $\endgroup$ – JMLCarter Sep 7 '17 at 20:50
  • $\begingroup$ Many argue that using the Schwarzschild coordinates around the event horizon leads to confusion, because they become singular and therefore non-applicable. Check out this explanation of how things actually fall to the singularity in a finite time: physics.stackexchange.com/questions/82678/… $\endgroup$ – safesphere Sep 7 '17 at 21:17

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